submodule--helper: move "config" to a test-tool

submodule--helper: move "config" to a test-tool

As with other moves to "test-tool" in f322e9f51b5 (Merge branch
'ab/submodule-helper-prep', 2022-09-13) the "config" sub-command was
only used by our own tests.

It was last used by "git submodule" itself in code that went away with
a6226fd772b (submodule--helper: convert the bulk of cmd_add() to C,
2021-08-10).

Let's move it over, and while doing so make it easier to reason about
by splitting up the various uses for it into separate sub-commands, so
that we don't need to count arguments to see what it does.

This also has the advantage that we stop wasting future translator
time on this command, currently the usage information for this
internal-only tool has been translated into several languages. The use
of the "_" function has also been removed from the "please make
sure..." message.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>